The present invention is directed to a method for controlling a countercurrent liquid-solid contactor and, more particularly, for more accurately controlling a countercurrent liquid-solid contactor so that dilution and contamination of the various product streams is minimized and the desired separation, concentration or conversion reactions are performed at peak efficiencies.
In all of the present day semi-continuous liquid-solid countercurrent, dense bed contactors, a hydraulic pulse of some fluid, e.g., water, is applied periodically in various ways to the solid phase in order to move the solid incrementally in a direction which is counter to the direction of the liquid phase movement; see Higgins, U.S. Pat. No. 2,815,322. The periodic movement of increments of the solid phase has the net effect of establishing in a given liquid-solid contacting zone, a set of concentration profiles, one in a liquid phase and one in a solid phase. There is always found at least two distinct liquid phases in the absorption and regenerating sections of the liquid-solid contactor. These phases may be entirely different solutions or they may be common solvent solutions of different concentrations with respect to some chemical constituent common to both solutions. In many instances these various liquid phases must be separated from one another in order to effectively accomplish the desired chemical or physical separations. The various liquid phases in a contacting zone must meet one another at more or less well-defined interfaces. They cannot be allowed to intermix. The only intermixing of the separate liquid phases that can normally be tolerated is the minute amount of intermixing which occurs as a consequence of diffusional mass transfer across the interfaces. Gross intermixing can and does cause excessive dilution of product streams, loss of regeneration because of regenerant dilution within the contactor, undesirable contamination of product phases, unfavorable chemical kinetics and loss of valuable chemical constituents.
The use of hydraulic pulse of fluid for pushing the solid phase within the contactor introduces some serious problems with respect to the maintenance of stable interfaces between the various liquid phases present in the contactor. The interfaces must be stable with respect to the degree of intermixing that occurs at the interface and with respect to their relative positions within the contactor. If a liquid-liquid interface were allowed to wander indiscriminately within a contacting zone, the advantages gained from the multi-stage countercurrent mass transfer operation would be lost.
Higgins, U.S. Pat. No. 3,579,322 describes the use of conductivity probes and metal-hydrogen detectors within the stripping and loading sections of an ion exchange contactor. These types of controls fail to give as effective and adequate interface positional control as that of the present invention. This failure is due to the fact that these prior art controls depend upon sharp, horizontal interfaces in order for the probes to continually sense and control at the same fixed point within the contactor.